This invention relates to self-lubricating bearings and particularly to self-lubricating bearings of nuclear fuel handling equipment.
In nuclear reactor systems well known in the art, the reactor vessel has a closure head sealed to the top thereof and fuel assemblies disposed therein in an array generally referred to as a core. The fuel assemblies consist of cylindrical fuel elements containing nuclear fuel which when appropriately arranged in the core produce heat in a manner well understood by those skilled in the art. The reactor vessel also has an inlet and an outlet for circulating a coolant in heat transfer relationship with the fuel assemblies thereby cooling the fuel assemblies. The coolant having absorbed heat from the fuel assemblies may then be conducted to a remote location to produce steam in a commonly understood fashion. After a period of reactor operation, the nuclear fuel in the fuel elements of the fuel assemblies becomes depleted or spent. Therefore, after a period of reactor operation, the spent fuel assemblies must be removed from the core and replaced with fresh ones in a process referred to as reactor refueling.
In many designs of reactor refueling equipment a grapple attached to a carriage located in a guide tube may be used to remove a chosen spent fuel assembly from the core or to place a fresh fuel assembly in the core. In these designs, the carriage may have wheels that slide on tracks in the guide tube which assures proper alignment of the grapple, carriage, and fuel assembly while the fuel assembly is transported. When lifting or lowering such a fuel assembly, the carriage may be required to be submerged in the reactor coolant pool while at other times the carriage may be at an elevation above the top of the reactor coolant pool. Since a cover gas may fill the void between the top of the reactor coolant pool and the bottom of the closure head, the carriage and its wheels become exposed to cyclical submergence and emergence from a coolant to a gas atmosphere. This cyclical exposure can have adverse effects on the bearings used in the wheels of the carriage that may severely limit the useful life of such a grapple and carriage. Such adverse effects are particularly prevalent in fast breeder reactors that utilize liquid sodium as the reactor coolant because the liquid sodium develops dross, which may be due to oxidation of the liquid metal or other impurities therein, on the top of the coolant pool at the liquid-gas interface as is well understood in the art. The dross may then seep into the small clearance in the bearings where it may cause the bearing to jam and, therefore, malfunction. Of course, this problem may be minimized by minimizing the number of times the bearing must pass through the layer of dross on the surface of the coolant; however, this is not a satisfactory solution where, due to other design criteria, minimizing such cyclical motion is not possible.
In U.S. Pat. No. 2,976,089 to R. B. Vogt, issued Mar. 21, 1961 there is described a roller wheel bearing that contains a fitting for forcing grease through the bearing so that the grease will exude through clearances in the bearing to thereby prevent entry of foreign particles. However, the Vogt patent does not solve the problems of preventing entry of such foreign matter where access to such bearing is limited because of the radioactive nature of the environment or where grease may not be used because of its contaminating nature. Furthermore, the Vogt patent does not describe a bearing that is capable of automatically replenishing the lubricant supply during normal operation. For these and other reasons, the roller wheel bearing described in the Vogt patent would not be suitable for reactor refueling operations or other operations wherein the bearing would be exposed to the dross formation.
A bearing device having an oil reservoir for supplying oil to bearings in rotary machines is disclosed in U.S. Pat. No. 3,804,476 to Nakamura et al., issued Apr. 16, 1974. While the reservoir described therein does provide an oil lubricant for the bearing, it is not capable of automatic refilling during the normal operation of the bearing. Moreover, the Nakamura patent describes a bearing which has the purpose to limit the flow of oil from the bearing and from the reservoir without addressing the problem of preventing foreign material from entering the bearing.